ARM: exynos_defconfig: Enable bus frequency scaling with devfreq

This patch enables the bus frequency scaling driver with devfreq and
devfreq-event framework.  Devfreq is already supported on Odroid U3/XU3,
Trats2 and Rinato/Monk boards.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
[k.kozlowski: Don't enable symbols already selected, adjust commit msg]
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>